The present invention relates to electric double-layer capacitors (EDLCs), which are sometimes also referred to in the art as “electrochemical capacitors”, “supercapacitors”, or “ultracapacitors”. More particularly, it relates to the use of organosilicon electrolytes, and in some instances optionally an organosilicon separator and/or binder in such devices.
A schematic example of a known type of EDLC is depicted in FIG. 1. This drawing shows an electrochemical double-layer capacitor 10 having two electrodes 11 which are kept from electrical contact by a separator 12. There are current collectors 13 at opposite ends of the device. The electrode consists of a porous material 14 and an electrolyte 15. Both the separator 12 and the porous material 14 are immersed in the electrolyte 15.
The electrolyte allows ions to move freely through the separator. The separator is designed to prevent electrical contact between the electrodes which otherwise might create a short circuit in the device.
The current collecting plates 13 are in contact with the backs of the electrodes 11. Electrostatic energy is stored in polarized liquid layers, which form when a potential is applied across two of the electrodes. A “double layer” of positive and negative charges is formed at the electrode-electrolyte interface.
Electrochemical double-layer capacitors provide energy storage as well as pulse power delivery. This is useful in many applications, and particularly where high power pulses are desired. In this regard, other energy storage devices such as batteries or fuel cells, which can store large amount of energy, cannot deliver high power pulses. Another advantage of supercapacitors is that they can be charged rapidly.
To be optimally effective such devices must, among other properties, have low internal resistance, store large amounts of charge, be physically strong, be stable at desired voltages, and be otherwise compatible with the usage environment. Therefore, there are many design parameters that must be considered in construction of such devices.
Electrochemical double-layer charge storage is a surface process and the surface characteristics of the electrode material can greatly influence the capacitance of the device. In this regard, the electrodes of a supercapacitor are typically made of very high surface area materials (e.g. porous carbon or carbon aerogels, carbon nanotubes, carbon foams or fibers, porous metal oxides) in order to maximize the surface area of the double-layers. High electrode-electrolyte interfacial surface area and nanometer dimensions of the charge separation layer result in high specific capacitance of electrodes so that high energy densities can be achieved in EDLCs.
It is particularly desirable that electrode materials in EDLCs have large pore diameters and good pore connectivity, so that electrolyte can easily penetrate the pores, facilitating rapid ion motion and high conductivity. Electrons can then easily flow from the electrode to the current collector and vice versa.
Carbon is the most widely used high surface area electrode material for EDLCs. Carbon can form specific textures, providing high surface area. High surface area carbon is particularly desirable because it can form mesopores or graphite crystallites suitable for ions intercalation.
See also U.S. Pat. Nos. 5,963,417 and 6,721,168, as well as U.S. patent application publication 2003/0030963 regarding a variety of other known high surface area materials for electrodes for various applications. The disclosure of these patents and patent application publications, and of all other publications referred to herein, is incorporated by reference as if fully set forth herein.
Aqueous and some organic electrolyte solutions have in the past been used in electrochemical double layer capacitors. Aqueous electrolytes, as compared to earlier organic electrolytes, provide lower equivalent series resistance improving the time constant of a supercapacitor and providing high power densities. However, they were not stable at the operating voltages exceeding the electrolysis voltage of water (1.23 V) and that made organic liquid electrolytes preferable to aqueous electrolyte solutions for many commercial applications.
Organic liquid electrolytes that can be used in supercapacitors should preferably have higher ionic conductivity. As an example, acetonitrile providing high ionic conductivity has been used in such electrolytes. However, acetonitrile is a hazardous flammable and toxic material, which produces highly toxic products (HCN and CO) upon combustion and thermal decomposition.
Other previously used organic liquid electrolytes, like those based on alkyl carbonates (ethylene carbonate, propylene carbonate, and γ-butyro-lactone, or dimethylcarbonate, diethylcarbonate, and ethylmethylcarbonate, for example) are highly flammable. They have lower ionic conductivity as compared to aqueous electrolytes or electrolytes based on acetonitrile, and this causes higher internal losses of stored energy and power density of the supercapacitor.
In unrelated work ion-conducting organosilicon polymer electrolytes have been proposed for lithium-polymer battery applications. See e.g. U.S. Pat. No. 6,337,383 and U.S. patent application publications 2003/0198869, 2004/0197665, 2004/0214090 and 2004/0248014. However, there was no teaching in these references to combine an organosilicon material with a high surface area material to create a supercapacitor electrode, or any suggestion to use ion-conducting organosilicon material as a binder or as a separator layer in supercapacitors.
Hence, there is a continuing need for EDLCs with improved safety and other characteristics, particularly those capable of operating stably at higher voltages.